This invention relates to a flaw detection system for the detection of flaws on a moving web of material, and more particularly to a method and apparatus for such a system which tracks the lateral edge movements of the web by generating a precision ramp analog voltage precisely related to the position of the scanning beam on the web of material, sampling and holding the various levels of the precision ramp voltage at predetermined time intervals for deriving edge position outputs which are converted to digital active scan pulses which accurately follow any lateral movement of the web.
U.S. Pat. No. 3,900,265 entitled Laser Scanner Flaw Detection System and U.S. Pat. No. 3,980,891 entitled Method And Apparatus For A Rotary Scanner Flaw Detection System which are both assigned to the assignee of the present invention, disclose flaw detection systems of the general type embodied in the present invention. In the aforesaid systems, flaws are detected on the surface of the material being examined by repetitively scanning a suitable light source, such as a laser beam, across the moving web of material. The laser light is reflected, transmitted, or scattered from the material, depending upon the characteristics of the material which light is picked up by a receiver having a suitable detector, such as a photomultiplier tube. At any instant of time during the scan, the photomultiplier output varies with the reflectivity, transmissivity or scattering properties of the spot of light on the material on which the laser beam is impinging, and deviations from normal variation provides a means for indicating flaws in the material.
In the prior art systems, whether a scanning mirror utilizing a galvanometer drive or a multi-faceted mirror drum is used for scanning in accordance with the aforesaid patents, a digital signal is derived for each scan line by a photoelectric or a magnetic pick up which provides a reference mark for each scan line. This reference mark, which has been referred to as a "once-per-facet" pulse, is used to synchronize counting and timing circuits driven by a high frequency crystal oscillator to establish the active scan intervals on each scan line. The active scan digital pulses from the timing circuits have a fixed time relationship with the reference mark during each scan and as such the digital pulses generated to determine the active scan interval have angular or positional variations depending on the speeding up or slowing down of the scanning mirror. Thus, the digital pulses previously used to establish the active scan interval on the material did not bear an accurate angular or positional relationship with respect to the position of the light beam on the web during an active scan interval. Furthermore, if the web of moving material jittered or was displaced laterally as it passed under the scanning laser beam the active scan interval, as defined by the timed digital pulses, would not follow such lateral movement. Even if some means were provided for the digital pulses to track the edges, the accuracy of the system would still be limited to the resolution which could be provided by the number of digital pulses generated during an active scan interval. To improve resolution using digital timing would require counter and clocking circuits of considerably higher speeds requiring more hardware and considerably more expense.
In some applications for flaw detection systems, such as the examination of coated webs, it is desireable to be able to set margins so that the edges which are not coated are not counted as defective portions of the web. The speeding up and slowing down of the speed of the scanning motor in systems such as illustrated in the aforesaid patents would cause errors in providing such margins. Also, margin setting resolution was limited by the number of digital pulses generated per scan.